1. Field of Invention
The present invention relates to a positioning device that determines its own position, to a positioning method, and to a timepiece having the positioning device.
2. Description of the Related Art
GPS (global positioning system) receivers are positioning devices that use signals from orbiting satellites to determine the position of the GPS receiver (device).
GPS devices receive signals from four or more GPS satellites to determine the position of the GPS device. More specifically, the GPS device selects four or more GPS satellites from among the constellation of GPS satellites orbiting the Earth and receives signals from the selected GPS satellites to determine the position of the GPS device.
The signals from the GPS satellites include orbital information describing the precise orbit of and information about the status of the GPS satellite that is transmitting the signal (called “ephemeris”), and information about the orbits of all GPS satellites in the constellation (called the “almanac”). The ephemeris and almanac are transmitted in a “navigation message.”
The same almanac is transmitted by all of the GPS satellites and is transmitted in segments due to the large amount of transmitted data. More specifically, as shown in FIG. 12, the navigation message is transmitted in one frame containing five subframes. The first three subframes contain clock correction data and high accuracy ephemeris, that is, the detailed orbital information describing the precise orbit and status of the GPS satellite transmitting the signal, and subframes 4 and 5 contain the almanac, that is, the information about the orbits of all GPS satellites. The almanac is further divided into pages 1 to 25. The almanac pages are sequentially transmitted in subframe 4 and subframe 5 until all of the information for one navigation message has been transmitted.
A Coarse/Acquisition (C/A) code composed of 1023 chips with a value of 1 or 0 is transmitted every 1 ms on a 1574.2 Hz carrier wave from the GPS satellite, and is transmitted superimposed with the navigation message. A unique C/A code is assigned to each GPS satellite, and the C/A code enables the GPS receiver to identify which GPS satellite transmitted the data.
The GPS receiver generates a signal of the same content as the C/A code assigned to each GPS satellite to synchronize with the signal from the GPS satellite and measure the transmission time from each GPS satellite while demodulating the navigation message from each GPS satellite to acquire the data from the GPS satellites.
It takes 30 seconds to transmit one complete frame of the navigation message, and it therefore takes 12.5 minutes to acquire all 25 pages of the almanac. A problem with this is that continuously receiving the GPS signals increases power consumption by the GPS receiver and the power supply of the GPS receiver therefore does not last long.
To solve this problem, Japanese Patent 3744180 (JP11-237462) (paragraph [0011]) teaches a GPS receiver that reduces power consumption by setting a time once a day to receive a certain subset of non-consecutive almanac pages at a 30 second delay and rewrite the internally stored data with the data transmitted from the GPS satellite over a number of reception cycles spanning plural days.
As shown in FIG. 13, however, if the GPS receiver taught in Japanese Patent 3744180 receives one set of almanac pages one day and the user travels a significant distance to a different location before the GPS receiver receives the rest of the almanac pages the next day, the almanac data received before traveling may be rendered invalid. This makes it difficult to use this GPS receiver in devices, such as timepieces, that may travel great distances in a short period of time.